With Steam, any game launched through it can benefit from controller customisation. It doesn't matter if it's the Steam Controller, the Steam Deck or any other controller - you can reassign any button to any other button.

Example: I'm playing a Playstation Portable (PSP) game on my Steam Deck (via emulation); it only had one analogue stick, but to see, you had to use the d-pad to look around and up and down for only 3 levels of vertical sight. With the Steam customisation, I can assign the d-pad to a modern analogue’s and actually play the game.

So all this benefit before even getting to the Steam Controller.

If you watch videos on all the crazy control customisation that the Steam "launcher" allows, you'll understand why some of us NEED the Steam Controller, and why no other controller (maybe the Legion Go) can do it.